Recently, in mobile communication, high-speed access and throughput performance have been improved as distribution of contents such as internet access, streaming broadcast, music, and video has increased as contrasted to voice-based communication by, for example, a conventional mobile phone.
In mobile communications, a method for allocating a relay station in a moving body has been considered as an improvement plan for the case where a user is on board the moving body where quality and throughput cannot be guaranteed by utilizing only a fixed wireless base station.
For handover processing of a system using a mobile relay station, as discussed in, for example, “handover method” (Japanese Laid-Open Patent Publication No. 2005-354448), a method for starting the handover processing between a fixed access point and a mobile relay station based on a travelling direction and speed of the moving body is disclosed. Further, “Automatic Radio Wave Output Limiting System In Mobile Phone” (U.S. Pat. No. 3,252,825) discusses user terminal inhibit processing in a speech inhibit area in the moving body.
FIG. 14 is a configuration diagram of a communication system 10 using a mobile relay station 40. As shown in FIG. 14, the mobile relay station 40, mounted on a moving body 50 such as a train, relays the communication between a mobile communication terminal 30 on the moving body 50 and a fixed wireless base station 20. A communication between the (fixed) wireless base station 20 and the mobile relay station 40 is called a relay link communication. A communication between the (fixed) wireless base station 20 and the mobile communication terminal 30, or between the mobile relay station 40 and the mobile communication terminal 30 is called an access link communication. Further, the transmission from the (fixed) wireless base station 20/the mobile relay station 40 to the mobile communication terminal 30 is called a forward link communication. Reversely, the transmission from the mobile communication terminal 30 to the (fixed) wireless base station 20/the mobile relay station 40 is called a reverse link communication.
Generally, in mobile communication, as the mobile communication terminal 30 moves, the handover processing occurs between the fixed wireless base station 20 and the mobile relay station 40. The handover processing is started by monitoring, mainly by the mobile communication terminal 30, a received signal from each of the fixed wireless base station 20 and the mobile relay station 40, respectively and detecting that a condition for handover processing is satisfied.
Further, as shown in FIG. 15, when a user of the mobile communication terminal 30 connected to the mobile relay station 40 gets off the moving body 50 and then the moving body 50 starts moving, communication quality deteriorates rapidly because of the movement of the mobile relay station 40 even though the movement amount of the mobile communication terminal 30 is small.
Further, the coverage area of the mobile relay station 40 may be generated around the moving body 50 because of radio wave leakage from inside the moving body 50. The coverage area moves together with the mobile relay station 40. As depicted in FIG. 16, when the mobile relay station 40 passes near the mobile communication terminal 30 that is not moving together with the mobile relay station 40, transmission quality to the mobile relay station 40 temporally becomes better than that to another fixed wireless base station 20 or another fixed relay station, and then a communication link may be established between the mobile relay station 40 and the mobile communication terminal 30. However, the mobile relay station 40 quickly moves away from the mobile communication terminal 30. Accordingly, the mobile communication terminal 30 has to be connected to the original fixed wireless mobile station 20 or other relay station by again performing handover processing. This series of procedures places extra load on the mobile communication terminal 30, the mobile relay station 40, and the fixed wireless base station 20. Further, a new connection to the mobile relay station 40 may be made because the power of the mobile communication terminal 30 that is moving with the moving body 50 together with the mobile relay station 40 is turned on, or because the mobile communication terminal 30 boards the moving body 50 while being connected to the fixed wireless base station 20 and then a handover to the mobile relay station 40 is required during the movement of the moving body 50.